In short: getting and installing SSL/TLS certificates made easy.

Certbot is a tool to automatically receive and install
X.509 certificates to enable TLS on servers. The client will
interoperate with the Let's Encrypt CA which will be issuing
browser-trusted certificates for free.

It's all automated:

The tool will prove domain control to the CA and submit a CSR
(Certificate Signing Request).

If domain control has been proven, a certificate will get issued and
the tool will automatically install it.

WWW: https://github.com/certbot/certbot